A complete, albeit brief review of the history of atoms and atomic-scale microscopy is offered. From the concept of the atom developed by Greek philosophers to the ultimate microscopy, the path of development is examined. Atomic-Scale Analytical Tomography (ASAT) is cited as the ultimate microscopy in the sense that the objects, atoms, are the smallest building blocks of nature. The concept of atoms developed as the scientific method grew in application and sophistication beginning in the Middle Ages. The first images of atoms were finally obtained in the mid-twentieth century. Early field ion microscopy evolved eventually into three-dimensional atom probe tomography. The crucial role of the electron microscope in atomic-scale microscopy is examined. Recently, combining atom probe tomography and electron microscopy has emerged as a path toward ASAT. The chapter concludes with the point that ASAT can be expected in the next decade.

This chapter begins with a formal definition of Atomic-Scale Analytical Tomography (ASAT) and the origins of the concept. The progression of experimental atomic-scale microscopies that led to ASAT concepts is reviewed, and the people and projects are highlighted. Once ASAT is established as a concept, its implications for structure-properties microscopy, coupled through Integrated Computational Materials Engineering (ICME), become obvious. A forward-looking roadmap for ASAT considers what length scales and atom counts in ASAT images are needed to address important microstructural questions. The chapter concludes with the notion that microscopy is at an inflection point: having reached the ultimate building blocks, the drive to see smaller and smaller must now evolve to a drive to see more and more of a structure.